Following a link from Adviso I came across another article that explains how changing a button can increase revenues by $300 million. The form used was the simplest possible with the strict minimum. The general opinion and mine too until reading the article is that it is so simple they can't have issues with the form:
We were wrong about the first-time shoppers. They did mind registering. They resented having to register when they encountered the page. As one shopper told us, "I'm not here to enter into a relationship. I just want to buy something."
The designers fixed the problem simply. They took away the Register button. In its place, they put a Continue button with a simple message: "You do not need to create an account to make purchases on our site. Simply click Continue to proceed to checkout. To make your future purchases even faster, you can create an account during checkout."
Not exactly the same, but it reminds me of sites that offer PayPal as a means to pay in France. Via PayPal you can use your Amex whereas these sites that allow you to use PayPal don't offer you the ability to pay using your Amex. Furthermore when you choose PayPal and look at creating an account via the PayPal for France version there is no option to use Amex (well there wasn't last time I tried).